1. Field of the Invention
The present invention relates to pumps used to transfer measured quantities of molten metal from a furnace to a die casting machine, ingot mould, DC caster or the like. In particular, the present invention relates to positive displacement pumps which are used with high melting point metals such as molten magnesium and its alloys.
2. Prior Art
There has been a steady increase in the use of magnesium and its alloys in the die casting industry especially as it relates to the automotive manufacturing sector. An explanation for the increase is that castings made of magnesium, in comparison to steel and aluminum, are lightweight but have equal or superior strength characteristics.
Special precautions and handling requirements must be taken with molten magnesium (which term shall be used hereinafter to include its alloys). The molten metal is highly volatile and bums when exposed to the atmosphere requiring the use of inert gases such as Argon or toxic gases such as sulfur tetrafluoride (SF6) and sulfur dioxide (SO2) to eliminate burning. The high melting point of magnesium also poses special considerations in designing a reliable pump.
There are various types of pumps available for delivering measured quantities of molten metal to die casting machines. Gas displacement, electromagnetic, centrifugal and positive displacement pumps are the general types of pumps used in the industry. The die casting machine receives a measured quantity of molten metal known as a shot which is used to make a casting. Both the quantity or metal delivered and the reliability of the delivery mechanism as described hereinafter are most important for reliable manufacturing casting operations.
Typically, the pump operates in conjunction with a transfer tube to deliver a shot to the die casting machine. As such the pump transfers a quantity of molten metal from the magnesium melt furnace into the tube for delivery to the die.
Gas displacement pumps use gas pressure to displace molten metal from a sealed vessel into a heated transfer tube which is submerged in the molten metal. Typically an inert gas is used to displace the molten magnesium through the transfer tube. However, this arrangement does not deliver sufficiently accurate shots. One reason for this is that the gas medium used to displace the molten metal by means of compression itself depends on temperature for its compression efficiency. As well, over time build up of magnesium occurs inside the tube creating increased resistance to the movement of molten magnesium through it. In addition, these pumps contain internal valves that are prone to clogging and are difficult to replace.
Electromagnetic pumps such as helical magnetic pumps, centrifugal electromagnetic pumps, flat linear induction pumps, and annular linear induction pumps use a magnetic force to induce current flow in a conducting fluid. The reaction to the flux causes a force to be exerted on the fluid so that pumping action takes place. Such pumps have no moving parts and can usefully transfer large volumes of metal but they are also inefficient and expensive.
For example, U.S. Pat. No. 5,407,000 to Mercer, II et al., discloses the use of an annular linear induction electromagnetic pump as part of a method and apparatus for handling molten metals. This pump is expensive and its accuracy depends on the viscosity of the molten metal which is variable. Temperature changes in the holding furnace affects the viscosity of the molten metal and ultimately, the size of the shots.
Centrifugal pumps comprise a cylinder with an internal rotating impeller which produces the necessary head pressure. Such pumps are typically used for transferring large quantities of molten metal but are incapable of delivering accurate small quantity shots.
Positive displacement pumps typically use a piston or plunger to displace and move the molten metal. One problem with this arrangement is that molten metal enters the space between the piston and the containing wall contributing to the clogging and unserviceability associated with this type of pump.
As well, most of the above mentioned pumps contain valves, guides and bearings which come in contact with the molten metal leaving them prone to failure and damage.
An object of this invention is provide a pump for transferring molten magnesium to a die casting machine that is simple in construction, effective, easy to use and maintain, and is relatively inexpensive to manufacture.
A further object of the invention is to provide a pump which does not have any valves, guides, bearings or the like, in contact with molten magnesium, thus making the pump less vulnerable to failure and more reliable.
A further object of the invention is to provide a pump which constantly agitates the molten metal in the magnesium melt furnace. The agitation assists in keeping a constant bath temperature in the furnace when ingots are introduced into the bath. Maintaining a constant temperature in the bath improves consistency and aids in preventing the premature release and deposits of magnesium metal alloys.
Agitation in the molten bath also assists in stabilizing the viscosity of the molten metal. Maintaining a consistent viscosity improves both pour speed and pour. Pour time relates to the amount of time required for the pump to change from idle to pour speed. The agitation is achieved by the auger which continues to rotate at idle speed to maintain a head in the transfer tube as described hereinafter. Naturally, at pour speed, the agitation is further increased. The agitation stabilizes the bath""s temperature quickly, substantially reducing the amounts of alloy deposits and variations in viscosity.
In the present invention, a molten magnesium pump comprises an upper and lower portion. The upper portion consists of an inverter duty electric motor which is operatively connected to a PLC. The PLC controls output by the motor thereby controlling the pour speed and pour time of the pump. A coupler attaches the motor to the auger shaft. The shaft is centered and aligned between upper and lower portions. The shaft is held in position by various guide bearings located exclusively within the upper portion.
The lower portion comprises a cylindrical casing which is immersed in the molten magnesium bath. The top portion of the casing contains a gas inlet aperture in which gases such as argon, SF6 or SO2 are fed. Located near the base of the casing is an inlet aperture which permits the molten magnesium to enter the pump casing. As the auger shaft is rotated by the motor, displacing the molten magnesium downwardly and forcing it out of the pump and into the heated transfer tube for delivery to the die casting machine. At idle speed, the auger maintains a head of metal in the transfer tube and agitates the molten metal in the bath.
Further objects and features of the present invention will become apparent to those skilled in the art in the light of the following description and accompanying drawings